This invention relates generally to refrigeration and air conditioning systems, and is more particularly directed to a flow control device for regulating the flow of condensed refrigerant liquid between the condenser and evaporator of a system.
In a centrifugal or rotary-screw water-chiller refrigeration system, condensed refrigerant flows to a lower sump portion of a condenser from which it must then be metered out to an evaporator. A flow control unit ensures that only condensed refrigerant liquid passes to the cooler, i.e., flooded evaporator and blocks any flow of refrigerant gas from the condenser to the cooler. In other words, the control unit maintains a liquid seal during operation of the system to improve system efficiency.
One approach to this is described in U.S. Pat. No. 5,009,079 granted to Thomas M. Zinsmeyer Apr. 23, 1991. In that patent, the control device includes a standpipe that extends upwardly from an outlet of a condenser sump, with a number of vertical openings spaced about the cylindrical wall of the standpipe near its lower end. A ring-shaped float is fitted closely over the standpipe so as to close off the standpipe openings when the level of the refrigerant liquid is below a minimum level. Then when the refrigerant level rises, the float rises with it, and uncovers the openings. This allows more refrigerant to flow through to subsequent stages, including the evaporator or chiller.
An improved version of this is described in U.S. patent application Ser. No. 07/822,784, filed Jan. 21, 1992, U.S. Pat. No. 4,209,080, having a common assignee, where a cylindrical metering sleeve is disposed within the cylindrical standpipe. That arrangement avoids binding between the float and standpipe, which could impair operation of the refrigerant flow control device. In each of these the float is formed of a ceramic or glass foam material, for example a slurry of microballs held together by epoxy adhesive, or another alternative ceramic or glass closed-cell foam.
This construction has some disadvantages, which can lead to inefficient system operation. One problem is that the float itself is somewhat delicate and brittle, and thus must be very carefully handled during manufacture and installation. The float is also quite expensive to manufacture. Moreover, after an extended period of use, the closed cells or microballs of the float tend to crack, and many of them fill with the refrigerant liquid. This makes the float somewhat less buoyant as the float material ages, causing the device to sink below its original level. As a result, flow at an improper rate of refrigerant liquid occurs, resulting in improper machine operation.
For environmental reasons, modern refrigeration systems often employ alternative refrigerants, such as R22, which are regarded as less dangerous to the environment if the material escapes from the system. However, a refrigerant such as R22 requires much higher operating saturation pressures, as it is a much more dense refrigerant vapor than others in common use previously. There are two consequences: First, the use of a higher vapor density refrigerant reduces the amount gas bypass that can be tolerated then otherwise. Bypass of vapor into the evaporator can reduce efficiency and can increase parasitic losses. Second, because the refrigerant liquid is rather dense, the foam or glass microsphere float becomes much heavier as the cracked cells or spheres become filled with the refrigerant liquid. This causes the float to sink below its design operating level.
It is therefore an object of the present invention to provide an improved refrigerant flow metering device which can be employed reliably with a modern centrifugal chiller system.
It is another object of this invention to provide a refrigerant flow control device which functions consistently over its lifetime, limiting bypass of refrigerant gas without undesired restriction of the liquid flow between stages.
It is a still further object of this invention to provide a refrigerant flow control device that is durable and less expensive than previous designs, and yet achieves increased reliability.